Scientists explored how the penis bone evolved among animals and primates including humans - how it became lost or lacking to some animals and how it became larger to some.

The penis bone had been lost to human males during evolution as their mating practices also evolved.

We all know that humans don't have bones in their penis while their evolutionary relatives like the macaque and the bagobos do.

The bacula, the scientific term of penis bones evolved in mammals between 145 to 95 million years ago. It was present in common ancestors of all primates and carnivores. It appears to be present in mammals and primates around the world but varies in sizes.
For instance, a stump-tailed macaque has extremely long baculum around 5cm for its size. A mangabey which is a larger monkey has a baculum that is five times the size of a macaque's.
Scientists embarked on a study to understand how the bacula evolved across species and disappeared in some.

Enough said.

Kit Opie who led the study at University College London explained that the presence of bacula in primates is tightly correlated to the increased duration of intromission during intercourse, which means the amount of time the penis stayed inside the vagina.
It may sound kinky but in primates' polygamous sex culture, staying longer inside the vagina is a practical strategy. It means the longer the stay, the bigger the chance of impregnating the female at the same time keeping her away from competing males. The bacula which attaches at the tip of the penis rather than the base, supports the penis during sex and keeps the urethra open.
Humans may have lost the penis bones when monogamy became the dominant reproductive strategy 1.9 million years ago during the time of Homo erectus. Males found no need to stay longer inside which may also be in part when females toned down and stopped leaping to other males.